The present invention relates to improved cooling systems for devices that need to be actively cooled to function reliably and efficiently. A primary application of this invention is microelectronic chips, devices, or systems that generate large quantities of heat in small volumes, thereby requiring high heat flux removal techniques. Other applications include rapid quenching of metals, thermal control of mirrors, and cooling of lasers.
As microelectronics continue to develop, there is an increasing trend of performing more functions at a faster rate in a smaller package volume. The net effect is that more heat is generated and it must be removed from a smaller surface area for efficient operation and reliability. It is not only desirable to remove the heat, but also to control the chip temperature independently of ambient conditions. It is well known that in certain microelectronic devices, such as monolithic microwave integrated circuit (MMIC) chips, that by lowering chip temperature, greater throughput may be allowed without damaging the chip and more efficient amplification is possible. These chips find application in electronically scanned active aperture antennas and have potential application in next-generation, electronic warfare systems and other equipment requiring high performance, electronically-steered antennas.
Common heat removal techniques such as conductive heat transfer through the use of heat sinks, natural or forced convective heat transfer, or combinations thereof, limit chip temperature to temperatures slightly above ambient. Active cooling systems provide the flexibility to cool microelectronics to temperatures below ambient and provide high heat flux removal. While there are several high heat flux cooling techniques, the present invention focuses on spray cooling, the advantages of which will become apparent in the following discussion.
There have been numerous investigations into the use of spray cooling for microelectronic systems. In most cases, practical application or systems to control the temperature and spray of the issuing spray have been limited. This is partially due to the lack of understanding of the contributions from each of the mechanisms behind spray cooling that lead to an optimized spray for high heat flux removal.
The first condition for high heat flux spray cooling is atomization of the fluid spray. Prior to spray cooling, early studies were performed using liquid jet impingement, where a narrow jet of fluid is directed upon the cooling surface. Later studies confirm that a finely atomized spray increases the heat flux removal capability of the fluid. Atomized sprays provide a more equal distribution of heat flux thereby maintaining more uniform temperature distributions across the cooling surface and preventing localized hot spots. This in turn prevents burnout and allows for higher critical heat flux, i.e. the point where increasing the temperature difference between the cooling surface and the spray is no longer associated with an increase in heat flux. Additionally, spray cooling allows for lower flow rates for equivalent average heat flux, thus reducing cooling system size. Spray atomization occurs when the magnitude of aerodynamic disruptive forces exceeds the consolidating surface tension forces and stabilizing viscous forces.
A wide variety of spray atomizers exist. Pressure-type atomizers are the most compact and therefore beneficial for compact microelectronics cooling packages. Within pressure-type atomizers, plain orifice and simplex pressure swirl are the most compact, simplistic, and rugged. To enhance spray cooling, several atomization conditions including uniform spray, complete cooling surface coverage, minimal momentum losses, and minimum spray evaporation losses are preferred.
Plain orifice atomizers produce uniform, full cone sprays. Simplex (or pressure-swirl) type atomizers generally produce a hollow cone that can be modified to produce a full cone by using an axial jet or some other device to inject droplets in the center of the hollow conical spray pattern. Hollow cone sprays can potentially lead to burnout at the cone center, whereas, full cone sprays created with axial jets or injections typically produce a bimodal distribution of drop sizes with the droplets at the center of the spray being larger than those near the edge leading to burnout at the spray edge. To prevent burnout, the entire heat-producing surface must be covered. xe2x80x9cSimplexxe2x80x9d atomizers produce a spray angle that is highly dependent on pressure differential across the nozzle. Practical application would require moving the nozzle axially to maintain c overage with changes in pressure differential or a control system to maintain nozzle pressure differential regardless of other system parameters such as cooling load or heat rejection temperature. Plain orifice atomizers produce a more constant spray angle that is mostly dependent on fluid properties such as viscosity and surface tension as well as turbulence of the issuing spray and therefore do not require axial locational control. Axial momentum improves spray cooling heat transfer by holding a thin liquid layer on the cooling surface. This is particularly important in adverse-gravity environments that may be encountered in space cooling applications or aboard aircraft during flight maneuvers. Therefore, it is desirable to minimize axial momentum losses that are typically increased as the radial spray component increases (i.e. increased spray angle). Plain orifice nozzles have a narrower spray cone (typically 5 to 15 degrees) than simplex nozzles (typically 30 to 180 degrees) and therefore reduced axial momentum losses. Additionally, narrow cone sprays are less susceptible to evaporation due to mixing of ejected liquid droplets and entrained vapor, thus providing more evaporation due to impingement of saturated liquid on the hot surface to be cooled.
These atomization conditions lead to an associated spray property that also enhances spray cooling. Prior art approaches typically use a spray that is xe2x80x9cat or nearxe2x80x9d saturated liquid conditions. This allows for two-phase boiling heat transfer at the heated surface and takes advantage of the high heat transfer associated with latent heat of vaporization. Two-phase heat transfer is typically at least an order of magnitude greater than single phase heat transfer. However, the condition of xe2x80x9cat or nearxe2x80x9d saturated liquid is not optimal. If the spray fluid is slightly subcooled (i.e. only near saturated liquid conditions) then nucleation for boiling must occur at the heated surface. This means the entire process of creating a nucleation site, allowing bubble growth, and removing the bubble to allow new nucleation sites must occur at the chip surface after the liquid droplets impinge on the surface.
It is not only important to initiate nucleation prior to contacting the surface to be cooled, it is also important that a system control the temperature at which this nucleation occurs and a system be capable of accommodating varying heat loads at the desired temperature. For instance, while it is desirable to constrain a spray to a uniformly atomized spray, that is two phase and is issued at an adjustable temperature, practical controls to perform these functions must be incorporated into a complete cooling system. Atomization in pressure-type spray nozzles only occurs if the pressure differential across the nozzle is sufficient.
At very low pressure differentials the flow begins as a xe2x80x9cdribblexe2x80x9d or xe2x80x9cthin distorted pencilxe2x80x9d. At intermediate pressure differentials additional stages such as xe2x80x9conionxe2x80x9d or xe2x80x9ctulipxe2x80x9d stages may occur prior to fully developed sprays at higher differentials. It is doubtful that the early stages will provide adequate cooling and will most likely lead to premature burnout. Therefore, control of the spray must be incorporated in a practical design. Optimum chip operating temperature may be determined a priori or through the use of chip performance monitoring plus feedback. For instance, high power microwave amplifiers measure efficiency as output power divided by input power. If it is desirable to optimize efficiency, then that value may be fed back to the spray control mechanism until the highest efficiency is achieved. Chip performance may alternatively be measured as the maximum power than can be transmitted through a chip without damaging that chip. If the maximum power is known, then the spray controller may be altered until maximum power is achieved. Finally, a combination of these and other chip performance feedback signals may be used.
U.S. Pat. No. 5,220,804 describes a wide-angle spray cooling system. The apparatus uses a simplex nozzle to spray liquid onto the heated surface. A method of controlling the state of the spray fluid is not disclosed. Other than the spray chamber the remainder of the system is not disclosed.
U.S. Pat. No. 5,412,536, U.S. Pat. No. 5,831,824, and U.S. Pat. No. 5,907,473 all discuss ways of spray cooling electronic systems. In each of these cooling systems the condenser is part of the spray chamber and a liquid pump is used to supply the pressurized liquid to the spray nozzle. This type of system, typically called a xe2x80x9cpumped loopxe2x80x9d can not control spray temperature below that of ambient temperature. Further, other than U.S. Pat. No. 5,412,536, disclosing that the spray emitted from the nozzle should be saturated liquid (meaning a quality of zero), no method for controlling the spray is given. In our invention, the spray emitted from the nozzle will have some quality, that is it will not be a saturated liquid but rather a combination of saturated liquid and vapor. The fluid has a quality of greater than zero but less than one.
U.S. Pat. No. 4,912,600 and U.S. Pat. No. 5,183,104 disclose spray cooling systems that use a vapor compression cycle, thus allowing spray temperatures to be below that of the ambient or heat rejection temperature. In the latter patent, a liquid is used in the impinging jet, and the xe2x80x9ccooling liquid should be preferable at or near its vaporization temperature when it impinges on the hot objects.xe2x80x9d This approach did not realize that the fluid should be saturated with a quality greater than zero (and less than one), that is the inlet fluid enthalpy should be selected or controlled so that after the pressure drop in the orifice the fluid is nucleating (due to the expansion) before it contacts the surface or environment to be cooled.
In these patents, the spray cooling chamber therein does not fully vaporize the refrigerant, resulting in the need for a downstream evaporator to fully evaporate the refrigerant vapor prior to compression. In addition to the extra hardware of such a conventional configuration, the coefficient of performance of the cooling system is reduced because not all the potential cooling is utilized in cooling the objects to be cooled via the spray cooling. The cooling provided by the downstream evaporator is not available for spray cooling in this configuration and represents lost cooling potential. It does not utilize all the potential cooling in the spray cooling chamber or recognize how to eliminate the downstream evaporator. Likewise the former patent also requires the downstream evaporator to completely vaporize the working fluid prior to compression and again is configured to spray liquid on the object to be cooled, as reflected in the statement xe2x80x9cin operation spraying the liquid coolant on one or both sides of the silicon wafers . . . .xe2x80x9d
Both of these known systems use an evaporator external to the microelectronics cooling chamber to evaporate any remaining fluid and prevent liquid slugging at the compressor. In addition to the excess size and weight of the evaporator and compressor, this method is inefficient.
Therefore, in light of our discovery of the benefits of saturated two-phase spray cooling, as well as in view of the afore-mentioned shortcomings in the prior art, our invention has the following objectives.
One object is to provide a single or multiple component two-phase fluid spray for cooling equipment that requires high heat flux removal. As opposed to the prior art that uses subcooled liquid or saturated liquid sprays that develop boiling nucleation sites at the surface to be cooled, the current spray is a saturated two-phase spray that includes nucleation sites prior to impingement on the cooling surface, thereby increasing heat flux, and reducing the wall superheat.
Another object is to provide a nozzle, capable of producing a narrow cone angle spray, which atomizes the fluid with the primary momentum in the axial direction of the spray. A narrow cone angle improves the uniformity of heat flux across the cooling surface.
A further objective is to utilize the adiabatic nucleation in the nozzle to provide additional liquid acceleration and increase the momentum in the axial direction of the spray. Homogeneous nucleation due to the isenthalphic vaporization improves the uniformity of spray droplets and thus the uniformity of the heat flux across the cooling surface.
A still further object is to control the spray to ensure that it is atomized and saturated two-phase and therefore to gain the benefits of uniform evaporative spray cooling with nucleation sites already formed.
Yet another object is to provide a spray cooling configuration that does not require an additional downstream evaporator. The cooling chamber serves as the sole evaporator, and flow rate is controlled to match the load, thereby producing a more compact and efficient cooling system.
Another object is to control the vapor compression system so that spray phase and temperature are controlled independently, thereby optimizing electronics performance.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings herein.